How do I distinguish a single-hop BFD session and a multi-hop BFD session

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Static BFD sessions: When the session is created for the first time, a single-hop BFD session is bound to a local interface while a multi-hop session is not.

Dynamic BFD sessions: Query the routes to determine whether a BFD session is a single-hop session or a multi-hop session. BFD for Internal Border Gateway Protocol (IBGP) sessions are multi-hop sessions.

Other related questions:
How to distinguish a single-hop BFD session and a multi-hop one on the AR router
For static BFD sessions, when the session is created for the first time, a single-hop BFD session is bound to a local interface, while a multi-hop session is not. For dynamic BFD sessions, query the routes to determine whether a BFD session is a single-hop session or a multi-hop session. BFP for Internal Border Gateway Protocol (IBGP) sessions are multi-hop sessions.

Is the BFD Echo function applicable to multi-hop BFD sessions
The BFD Echo function is only applicable to single-hop BFD sessions. For two directly connected devices, one device supports BFD, and the other device supports only forwarding at the network layer and does not support BFD. To rapidly detect forwarding failures between the two devices, configure the BFD Echo function on the BFD-supporting device. The BFD-supporting device sends an Echo Request packet to the remote device, and the remote device sends the Echo Request packet back along the same path to detect the connectivity of the forwarding link.

For S series switches, whether the BFD echo function supports multi-hop BFD sessions
For S series switches (except S1700), BFD echo is applicable only to single-hop BFD sessions. Between two directly connected switches, one switch supports BFD, but the other does not support BFD and supports only forwarding at the network layer. To rapidly detect forwarding failures between the two switches, configure the BFD echo function on the BFD-supported device. The BFD-supported switch sends an Echo Request to the remote device, and the remote device sends the Echo Request back along the same path to detect the connectivity of the forwarding link.

Can a single-hop BFD session be established when a router directly connects to a PC
No, a single-hop BFD session cannot be established in this situation. This is because the PC does not have the IP packet forwarding capability and cannot forward BFD Echo packets.

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